Force sensors for determining the weight of vehicles on the road while they travel over these sensors, namely so-called WIM (Weigh-In-Motion) sensors, are installed in a roadway such as, for example, a road and measure the dynamic ground reaction forces of the vehicles in order to determine their weight based on these forces. Piezoelectric sensors are typically used for this purpose. A signal in the form of a charge is generated in this case. It is common practice to transmit the charge to an external charge amplifier that converts the charge signal into a voltage signal via a cable. Since the charges are very low, the signal path up to the charge amplifier needs to be realized in a highly insulating fashion as it is always required for lines of piezoelectric sensors. This results in high costs because the highly insulating cables are very expensive. Long cables are frequently required in this application because the cabinet containing the electronics may be located far from the sensor in the road such that the costs of the system increase accordingly.
FIG. 1 shows a schematic representation of a roadway 6 in the form of a road with two rows of WIM (Weigh-In-Motion) sensors 3 that are integrated into the road. Analogous arrangements for rail vehicles are also conceivable, wherein the sensors 3 are in this case arranged, for example, in the rails. WIM sensors 3 typically contain piezoelectric sensor elements 3′ (see FIG. 3) that deliver charge signals. These charge signals need to be transmitted via highly insulating cables 5 (see FIG. 1) before they are converted into voltage signals in corresponding electronics.
In the arrangement shown in FIG. 1, the WIM sensors 3 are used for determining the weights of the wheel loads of the vehicles traveling over the sensors. The measured results are transmitted to an evaluation unit in a cabinet 4 that is typically arranged very far from the road via a cable 5.
A charge amplifier 1 of the type normally used for such applications in the shown cabinets 4 is illustrated schematically in FIG. 2.
The sensor signal is fed to the charge amplifier 1 at the two inputs “In” and “GND” (ground) via a highly insulating coaxial cable 5 over a distance of several meters and converted into a voltage that can then be processed further in a relatively simple fashion (low-impedance signal processing). The charge amplifiers 1 are frequently accommodated in distant electronics cabinets on the side of the road.
The highly insulating connecting cable 5 from the sensor 3 to the charge amplifier 1 is very expensive and requires a very clean installation in order to prevent the cable 5 from being soiled and thereby compromising the required high insulation.
The cables 5 for transmitting the signals are connected to the two outputs “Out” and “GND.” In addition, the charge amplifier 1 needs to be supplied with power via a two-core cable 5 such that a total of four cables 5 are connected to the charge amplifier 1. Depending on whether the charge amplifier 1 is accommodated in the sensor 3 or in the cabinet 4, either an expensive highly insulating cable 5 or an expensive four-core cable 5 needs to be installed between the sensor 3 and the cabinet 4.
The transmission of a voltage signal is much less problematic than that of a charge signal. If the sensor signal is already converted into a voltage signal in the sensor, the subsequent handling is significantly simplified, for example, because a less complex cable can be used.
Two established circuits are available for the conversion of a charge into a voltage signal. The first option in the form of a so-called IEPE circuit (Piezotron® available from Kistler Instrument Corp. of Amherst, N.Y.) has the advantage that it requires few components. Since the electronics are installed in the roadway together with the sensor in the described application of WIM sensors, a malfunction must be prevented in all circumstances because the sensor cannot be readily replaced. In this case, the roadway is impassable for a certain period of time and needs to be blocked off. Further requirements with respect to the quality of the sensor arise from the high temperature fluctuations and other environmental influences expected at the installation site. A small number of components contributes to realizing the electronics in a simple and therefore more durable fashion. A second advantage of an IEPE circuit can be seen in that a normal coaxial cable suffices for the signal transmission and the power supply if the charge signals are already converted into a voltage signal in the sensor.
Such an IEPE circuit according to the prior art is disclosed in FIGS. 7 and 8 of U.S. Pat. No. 4,009,447, which patent is hereby incorporated herein in its entirety by this reference for all purposes. The sensitivity of the signal respectively changes due to the high capacitance of the sensor and due to the change of this capacitance during temperature fluctuations such that the measuring results become less accurate.
Another example of an IEPE circuit is described in U.S. Pat. No. 5,792,956, which patent is hereby incorporated herein in its entirety by this reference for all purposes, but the ground output of the sensor does not have the same potential as the ground output of the amplifier in this case because a Zener diode is inserted. However, since the sensor ground needs to have the same potential as the output signal ground in a WIM sensor, this circuit cannot be used for the described purposes.
FIG. 3 shows an IEPE circuit 2 that is also referred to as a Piezotron® circuit and may be typically integrated into a sensor 3 of the described type, particularly a piezoelectric sensor 3. Aside from the measuring element, the sensor capacitance Cs and the insulation resistance of the sensor Rp are indicated in the sensor 3. The cable capacitance is indicated with the reference symbol Ck.
The centerpiece of the IEPE 2 is the impedance converter T2 that is arranged on the output side. It is connected to a two-core cable 5′ (FIG. 3), into which a coupler is integrated for the power supply.
In both applications, no (quasi-)static measurements are required, which is the reason why both systems operate with time constants that result from the values of Cg and RT. In this way, a bottom limit of the frequency response is achieved.
In the IEPE circuit 2 schematically depicted in FIG. 2, the insulation resistance of the sensor Rp lies parallel to RT and therefore also influences the time constant.
The sensitivity of the measuring chain is in the IEPE circuit 2 defined by the capacitance Cg at the input, the sensor capacitance Cs and the input cable capacitance Ck.
In a WIM, the sensor capacitance Cs is high and, in particular, not stable over the service life due to temperature influences, manufacturing tolerances and other environmental influences. This massively affects the sensitivity of the measuring chain and practically makes it impossible to carry out an exact measurement.
The advantage of the IEPE circuit 2 can be seen in the simple signal transmission and power supply of the system by means of a cable pair 5′. The system is operated with a few mA by a power source and the signal is superimposed on a quiescent value (bias voltage) of ≈10 VDC in the form of a voltage value.
Nevertheless, an IEPE circuit cannot be used in the described application because the high capacitance and the internal resistance of the sensor are direct components of the circuit. However, the capacitance varies between the individual sensors such that each sensor has different signal characteristics. In addition, the resistance of each sensor changes over time such that the sensitivity of the signal also changes over time. An IEPE circuit consequently does not make it possible to obtain measuring results that have the required accuracy, particularly over a prolonged period of time.
The second option in the form of a charge amplifier does not have these disadvantages. However, a shielded two-core cable, which may be very long and therefore very expensive as described above, is required for the signal transmission and the power supply in this case. Consequently, this is not the desired solution. If the charge amplifier is directly installed into the sensor, an additional cable is required for the power supply such that the costs of the system are increased again.
Another charge amplifier according to the prior art is disclosed in FIGS. 10 and 11 of U.S. Pat. No. 4,009,447. In this arrangement, a voltage across the sensor is applied by a Zener diode, wherein this leads to a current flow through the sensor that has to be prevented by all means if the insulation deteriorates over time.